1. Field of the Invention
The present invention relates to a rectifier circuit, and particularly to a rectifier circuit for rectifying a voltage.
2. Description of the Related Art
At present, importance of rectifier circuits that operate at a low voltage is growing with a tendency toward compact and portable systems. Such rectifier circuits are used, for example, in IC cards or ID chips which cannot contain a battery as a power supply. In the IC cards or ID chips, an electric power is taken out from radio wave radiation energy, and voltage reduction is prevented by the rectifier circuits that operate at a low voltage, whereby a wider communicable range is realized. A diode or Metal Oxide Semiconductor (MOS) transistor that is turned on and off according to the magnitude relation in voltages between two terminals is used in the rectifier circuit (see, e.g., Udo Karthaus et al., “Fully Integrated Passive UHF RFID Transponder IC with 16.7-uW Minimum RF Input Power”, IEEE J. Solid-State Circuits, Vol.38, No. 10, p.1602-1608, October 2003, Yasuo Nantoh, “Electronic Circuit and Analogue IC”, p.174, Kougakutosho Ltd.).
FIG. 17 is a circuit diagram of a diode-connected NMOS transistor. As shown in the diagram, a gate and a drain are connected in an NMOS transistor M101. Herein, a voltage of the drain is designated as a voltage Va, a voltage of the source is designated as a voltage Vb, and a threshold voltage of the transistor M101 is designated as a threshold voltage Vthn. In this case, when the following formula (1) is satisfied, a current flows from the drain to the source.Va≧Vb+Vthn  (1)
More specifically, when the voltage Va of the drain is higher than the voltage Vb of the source by the threshold voltage Vthn, the transistor M101 is turned on and a current is allowed to flow. On the other hand, when the voltage Va of the drain is not higher than the voltage Vb of the source by the threshold voltage Vthn, the transistor M101 is turned off and no current is allowed to flow.
FIG. 18 is a circuit diagram of a diode-connected PMOS transistor. As shown in the diagram, a gate and a drain are connected in a PMOS transistor M102. Herein, a voltage of the source is designated as a voltage Va, a voltage of the drain is designated as a voltage Vb, and a threshold voltage of the transistor M102 is designated as a threshold voltage Vthp. In this case, when the following formula (2) is satisfied, a current flows from the source to the drain.Va≧Vb+Vthp  (2)
More specifically, when the voltage Va of the source is higher than the voltage Vb of the drain by the threshold voltage Vthp, the transistor M102 is turned on and a current is allowed to flow. On the other hand, when the voltage Va of the source is not higher than the voltage Vb of the drain by the threshold voltage Vthp, the transistor M102 is turned off and no current is allowed to flow.
In a diode, when a voltage of an anode is higher than one of a cathode by a forward voltage Vf or more, a current is allowed to flow.
As described above, since the threshold voltage for turning on a switch (transistor or diode) is present in the rectifier circuit, a conduction loss occurs during the rectification. This conduction loss can be improved by reducing the threshold voltage. Further, it means that accuracy of the determination of whether a switch is turned on/off at the input of a small signal can be improved by reducing the threshold voltage. For reducing the threshold voltage, a schottky diode having a low forward voltage and a MOS transistor having a low threshold voltage are used.
However, when configuring the rectifier circuit by employing a semiconductor integrated circuit, the following problems arise. The schottky diode or the MOS transistor having a low threshold voltage is realized by addition of process steps and as a result, costs are increased.
In addition, the diode or the transistor is different from other diodes or MOS transistors in the manufacturing process and therefore, variation in the devices is caused.